1. Field of the Invention
The present invention relates generally to incoherent demodulators, and more specifically to an incoherent demodulator having an improved converging performance at signal points and excellent stability against low-speed phase rotation.
2. Description of the Related Art
A prior art demodulator, as shown in FIG. 1, receives a QAM (quadrature amplitude modulated) IF signal from a down-converter, not shown, and includes a pair of analog mixers 21 and 22 in which the IF signal is mixed with orthogonal carriers. A local oscillator 41 produces an in-phase IF carrier that is supplied to the mixer 21. The in-phase IF carrier is xcfx80/2 phase-shifted in a phase shifter 42 to produce a quadrature IF carrier which is fed to the mixer 22. Mixers 21 and 22 perform multiplication between the received IF signal and the sinusoidal waveforms of the orthogonal IF carriers to detect in-phase (P) and quadrature (Q) baseband components for respective channels. Undesired high frequency components of the P- and Q-channel baseband signals are eliminated by analog low-pass filters 31 and 32, respectively.
Since the frequency of the local carriers is very close to, but not synchronized to, the frequency of the received IF signal, there exists a small phase rotation. In addition, variability inherent in the operating characteristics of these analog circuits causes the recovered baseband signals to vary in amplitude independently of each other with varying ambient temperature. As a result, there is a small power difference between the P- and Q-channel baseband signals.
The P and Q analog baseband signals from the low-pass filters are converted to digital baseband signals by A/D converters 61 and 62. The output signals of the A/D converters 61, 62 are supplied to a digital endless phase shifter (EPS) 80. Phase shifter 80 is comprised of a numerical controlled oscillator (NCO) and digital circuits. Using the digital orthogonal carriers, the endless phase shifter 80 removes the demodulator""s inter-channel rotating phase difference according to the following equations and produces output signals Pxe2x80x2 and Qxe2x80x2:
Pxe2x80x2=P*cos xcex8xe2x88x92Q*sin xcex8
Qxe2x80x2=P*sin xcex8xe2x88x92Q*cos xcex8
where xcex8 represents the phase angle which is controlled by the phase shifter 80 so that its output signals Pxe2x80x2 and Qxe2x80x2 would be exactly what is recovered if the locally generated orthogonal IF carriers were precisely synchronized to the received orthogonal IF carriers. More specifically, if the angle of phase rotation between the recovered P- and Q-channel baseband signals is equal to xcex8(t), the phase shifter 80 introduces a time-varying phase shift of xe2x88x92xcex8(t) to the input P- and Q-channel signals.
Similar to the power difference between the recovered P- and Q-channel baseband signals, there is a power difference between the baseband signals which is produced by the transmitter""s modulator as a result of the variability of its analog components. This modulator""s baseband power difference is eliminated by automatic gain control (AGC) circuits 91 and 92 connected to the outputs of phase shifter 80. Additionally, the AGC circuits 91 and 92 monitor their outputs to detect their offset values from prescribed levels and control their output levels so that they confirm to the prescribed levels.
The baseband power difference caused by the analog demodulator is reduced by a closed-loop AGC control circuit 70 formed by a difference detector 71, an integrator 72 and a multiplier 73. Difference detector 71 is connected to the outputs of AGC circuits 91 and 92 to detect a difference which exists between the output levels of these AGC circuits according to the phase rotation angle xcex8 detected by the phase shifter 80. The output of the difference detector 71 is integrated by the integrator 72 to produce a gain control signal, which is applied to the multiplier 73. The output of the integrator 72 is used in the multiplier 73 to control the amplitude of the P-channel input signal of the digital phase shifter 80 from the A/D converter 61. By the feedback operation of the closed loop 70, the baseband power difference between the recovered baseband signals reduces towards zero.
However, the inter-channel power difference of the demodulator cannot be completely eliminated by the closed-loop AGC control circuit 70. As a result, the demodulator suffers degradation in converging performance at signal points in the 16-QAM signal constellation. As shown in FIG. 2, due to the degraded convergence performance, the size of signal points in the 16-QAM signal constellation increases with distance from the center of the constellation. Hence, the bit error rate performance of the demodulator suffers degradation. In addition, if the inter-channel phase rotation varies at a considerably low speed (or near zero), the control signals of all AGC circuits 70, 91 and 92 will assume an equal value, causing the demodulator to enter an unstable state if these circuits have substantially the same time constant value. On the other hand, if the AGC circuit 70 has a greater time constant value than the AGC circuits 91, 92, the former ceases to function properly and the latter takes control to absorb the inter-channel power difference. When the phase rotation begins increasing its speed under such conditions, an error would result until the demodulator is stabilized. Similar problems occur in an interference canceller for cancelling interference between cross-polarizations.
It is therefore an object of the present invention to provide an incoherent demodulator that is improved in terms of convergence performance and stability against low speed phase rotation.
According to a first aspect of the present invention, there is provided an incoherent demodulator comprising local oscillator circuitry for producing a pair of orthogonal carriers, a pair of analog mixers for incoherently demodulating a pair of modulated orthogonal signals with the orthogonal carriers to produce a pair of analog orthogonal baseband signals, there being a phase rotation in the analog orthogonal baseband signals resulting from the incoherent demodulation of the modulated signals. A pair of analog-to-digital converters process the analog orthogonal baseband signals to produce first and second digital signals. Gain controlled circuitry is provided for scaling the first digital signal so that a difference which exists between average power of the scaled first digital signal and average power of the second digital signal reduces to zero. A digital phase shifter processes the first and second output signals of the gain controlled circuitry so that the processed first and second output signals no longer contain the phase rotation.
In a preferred embodiment, the gain controlled circuitry includes a digital multiplier for multiplying the first digital signal with a control signal to produce a scaled first digital signal, averaging circuitry for producing a first average value representing the average power of the scaled first digital signal and a second average value representing the average power of the second digital signal, and control circuitry for deriving a signal from the first and second average values and supplying the signal to the digital multiplier as the control signal so that a difference which exists between the first and second average values reduces to zero.
According to a second aspect of the present invention, there is provided an incoherent demodulator system for a cross-polarization communication system, comprising a common local oscillator for producing a pair of orthogonal carriers, a pair of first and second demodulators for receiving first and second cross-polarized signals, respectively, and an interference canceller for cancelling interference between the first and second cross-polarized signals. Each of the first and second demodulators includes a pair of analog mixers for incoherently demodulating a pair of modulated orthogonal signals with the orthogonal carriers to produce a pair of analog orthogonal baseband signals, there being a phase rotation in the analog orthogonal baseband signals resulting from the incoherent demodulation of the modulated signals. A pair of analog-to-digital converters process the analog orthogonal baseband signals to produce first and second digital signals. Gain controlled circuitry is provided for scaling the first digital signal so that a difference which exists between average power of the scaled first digital signal and average power of the second digital signal reduces to zero. A digital phase shifter processes the first and second output signals of the gain controlled circuitry so that the processed first and second output signals no longer contain the phase rotation. First and second automatic gain controlled circuits respectively process the first and second digital output signals processed by the digital phase shifter to produce amplitude-controlled digital output signals whose amplitudes are maintained at a predetermined value. The amplitude-controlled digital output signals produced by the first and second AGC circuits of the first demodulator are supplied to the interference canceller, and replicas of the amplitude-controlled digital output signals produced by the first and second AGC circuits of the second demodulator are supplied to the interference canceller as interference cancelling signals.
According to a third aspect, the present invention provides a demodulation method comprising the steps of incoherently demodulating a pair of modulated orthogonal signals with a pair of orthogonal carriers to produce a pair of analog orthogonal baseband signals, there being a phase rotation in the analog orthogonal baseband signals resulting from the incoherent demodulation of the modulated signals, converting the analog orthogonal baseband signals to first and second digital signals, scaling the first digital signal so that a difference which exists between average power of the scaled first digital signal and average power of the second digital signal reduces to zero, and removing the phase rotation which exists between the scaled first digital signal and the second digital signal.